1. Field of the Invention
The present invention relates to an autotensioner imparting a suitable tension to a timing belt of an automobile engine or a belt driving a plurality of auxiliary equipment.
2. Description of the Related Art
An autotensioner is employed in a belt drive mechanism for transmitting drive power of an engine to a plurality of equipment by a single endless belt. It imparts a suitable tension to the belt and causes the vibration of the belt caused by the fluctuation of the rotational speed and load of the engine to attenuate. Due to this, the drive force of the engine is reliably transmitted to the equipment.
In general, an autotensioner is provided with a cup-shaped base fixed to an engine block etc., a rocking arm rotatably axially supported at the base, and a pulley attached to the front end of the rocking arm and abutting against the belt. The rocking arm is rotationally biased in a direction tensing the belt by a torsion coil spring, which is housed in the base and is provided substantially concentrically with the center of rotation. Due to this, suitable tension is imparted to the belt. Further, a bushing or friction member is provided between the rocking arm and the base for frictionally sliding with at least one of the same. Due to the bushing, a frictional force forming a rotational resistance occurs when the rocking arm rotates relative with respect to the base, the rotation of the rocking arm is damped, and vibration of the belt is attenuated.
In recent years, along with the improved performance of engines, the fluctuation in the rotational speed of the engine and the load applied to the belt has increased. The fluctuation of tension of the belt has also become larger. Therefore, when the damping force is small, the fluctuation in tension of the belt cannot be fully suppressed, and this results in vibration together with vibration of the belt. Therefore, to improve the damping performance of the autotensioner, a higher damping force is required. In particular, it is preferable to make the damping force acting on the rocking arm when the belt is tense (first damping force) larger than the damping force acting on the rocking arm when the belt is slack (second damping force).
To answer this demand, some damping mechanisms provided with the friction member have been proposed. However, in the conventional damping mechanisms, it is difficult to make the first damping force much different from the second damping force, and if the twisting torque of the torsion coil spring is raised to increase the damping force, not only the first damping force, but also the second damping force increases and the timing of tensing of the belt becomes slower, that is, the problem arises of a fall in the ability to follow the belt.
Thus, in the conventional damping mechanisms, it was not possible to simultaneously satisfy both the demands of improving the damping performance of the autotensioner and maintaining the ability to follow the belt well.
On the other hand, in recent years, along with the increasingly smaller size of engines, autotensioners have been required to be made smaller in size as well. If for example the base is made smaller in size, the space where the torsion coil spring is housed becomes smaller, so the torsion coil spring which can be housed is also limited to a smaller one. On the other hand, along with the increasingly sophisticated performance of engines in recent years, autotensioners have been required to have a considerable biasing force. For an autotensioner having a considerable biasing force, a torsion coil spring having a considerable spring constant and a considerable spring biasing force is required. The spring constant and spring biasing force are determined by the coil length, wire diameter, etc. Therefore, for an autotensioner to have the necessary biasing force, a space able to house a considerable spring is required. In an autotensioner, in which the size of the base is reduced, it is not possible to house a torsion coil spring having a sufficient torque, and the biasing force to the belt may become insufficient.
Further, the friction member is required to be water resistant and to remain unchanged in frictional force even when sprayed with water, in addition to being superior in heat resistance, abrasion resistance, strength, and dimensional stability. In the past, a synthetic resin superior in heat resistance, for example, a nylon resin, has been used for friction members. A friction member made of a nylon resin, however, increases in frictional force with the rocking arm when exposed to water or salt water, so there was the problem that smooth rotation of the rocking arm was obstructed and abnormal noise occurred from the autotensioner or belt.